An important use of aqueous dispersions of anionic charged polymers is retention and dewatering aid in paper manufacturing industry. Further uses of such polymer dispersions are as aids in various processes where they act as, for example, flocculants when treating wastewater or aids in other solid-liquid separation processes in, for example, the metal, ceramic, printing, biotechnological, and pharmaceutical industries. They can also be used as thickeners in, e.g., chemical, biotechnological, pharmaceutical, and cosmetic industries and soil improving agents. Generally, these polymer dispersions comprise a dispersed polymer and a dispersant in which the dispersant usually is a polymeric dispersant.
Anionic polymer dispersions are generally prepared by polymerising a reaction mixture of water-soluble anionic and non-ionic monomers in the presence of a salt. Finished polymer will precipitate from the aqueous salt solution and, by using a suitable dispersant, form a stable polymer dispersion. WO 01/18063, U.S. Pat. Nos. 5,837,776 and 5,605,970 disclose processes for preparing a dispersion of a water-soluble polymer comprising polymerising water-soluble monomers in an aqueous reaction mixture containing a salt.
Factors to consider are, for example, the process viscosity, active content, stability, good retention properties, and easiness of preparing the polymer dispersion sometimes including preparing the stabiliser. Also, criteria such as environmental and safety aspects are of importance.
There are a number of criteria that the polymer dispersion should fulfil to give good results in the final application and be of commercial interest. Such criteria are for example low-cost production, rapid performance, effective flocculation or dewatering, and long shelf life.
Prior art anionic polymer dispersions described contain significant amounts of salt which makes up a great part of the weight of the polymer dispersion. There is presently a desire to reduce, or completely avoid, the use of salt in polymer dispersions due to environmental and economical reasons.
The viscosity of the reaction mixture when producing the polymer dispersion, “the process viscosity”, should be kept low and viscosity peaks should be avoided, or at least reduced as much as possible, during the production of the polymer dispersion.
The shelf life of the dispersion, i.e., the stability of the polymer dispersion over time, is an important property. An efficient dispersant is needed for keeping the polymer particles stable in dispersion without settling as sediment.
A further factor to consider is the active content, i.e., the amount of dispersed polymer in the polymer dispersion. A high active content minimises transportation costs and gives easier handling at the end-application. By using an efficient dispersant, dispersions with a high active content can be obtained at the same time the viscosity can be kept low. However, an increase of the active content above a certain level may not always give improved performance in retention and dewatering in a papermaking process.
During preparation of a polymer dispersion, deposits of polymer may form and stick to the reaction vessel and stirrer. This leads to time consuming cleaning procedures of the reaction equipment.
It is an object of the present invention to provide a, preferably salt-free, water soluble anionic polymer dispersion having high stability and high active content. The polymer dispersion should also give good retention and dewatering results when used in papermaking processes, act as a good flocculant in other processes such as waste water treatment, act as a good thickener in various applications such as cosmetic formulations and also be possible to use in soil improvement processes. It is further an object of the present invention to provide a process for preparing a, preferably salt-free, water soluble anionic polymer dispersion in which the process viscosity is kept low and smooth during preparation without any large viscosity peaks, and which gives no deposits. Finally, it is an object of the present invention to provide a process for producing paper in which the polymer dispersion is used.